There is a open bug [1] on the php tracker for this exact issue, but it doesn't look like they are gonna port this back to 5.6 (not even the other stable branches to be exact). The php package in the official repos got a patch [2] to detect freetype via pkg-config, maybe it can be backported to 5.6?

The PHP development team announces the immediate availability of PHP 5.6.36. This is a security release. Several security bugs have been fixed in this release. All PHP 5.6 users are encouraged to upgrade to this version.